In the battle of the belly, you’ve made big strides: You’ve stopped thinking of french fries as a vegetable, Ben & Jerry no longer occupy a permanent spot in your freezer, and you try to break a sweat a few times a week. But when it comes to ab exercises, are you getting the most you can from the moves or simply wasting your time? We asked exercise physiologist Amy Dixon, creator and star of the Breathless Body and Give Me 10 workout DVD series, for the low-down on the five worst ab exercises of all time—and how to fix them.

Barely-moving bicycle. Sure, the bicycle is an awesome ab move—when it’s done right. This exercise, in which you lie on your back with bent knees and use your stomach muscles raise one shoulder toward the opposite knee, can be easy to slack on. “It’s a waste of time to simply lie on your back and reach your elbow toward your knee because it’s not giving you enough rotation in your torso,” says Dixon. Instead, you want to twist fully through your ribcage and bring your shoulder (not your elbow) as close as you can to the opposite knee. (Put a little oomph in it!)

Neck-wrenching sit-ups. Even the name sounds painful, right? But Dixon sees far too many gym-goers doing sit-ups that fit the description to a T. “Pulling on the back of your head while doing a sit-up puts a lot of tension on your neck,” she says—and that could lead to injury. Plus, if you’re using your clasped hands to drag your shoulders off the floor, you’re not working the muscles you’re supposedly trying to tone. To fix it, touch your hands loosely to the back of your head instead of linking fingers, then tighen your ab muscles to lift you up, making sure your head and neck stay in line with your torso.

Speed crunches. OK, we get it–you just want to get through these. But faster isn’t always better, especially when it comes to crunches. “When you use momentum to perform a crunch, you aren’t engaging all of the muscles you should be,” says Dixon. And when those muscles get a free pass, you don’t get the results you want. The fix is simple: Slow down and focus on contracting your belly as you move.

Droopy-hip push-ups. You’ve got your hands under your shoulders, your toes tucked—but what’s happening with your hips? If they’re reaching for the floor, you’re not only missing out on the total core engagement that makes this such an effective move, “but your poor alignment could put a strain on your shoulders and neck, putting you at risk of injury,” says Dixon. For perfect form, try to keep your hips even with your shoulders. If that’s just not in the cards until you build up a little more strength, drop your knees to the floor in the meantime.

Cheater’s plank. “There are so many ways to make plank easier,” observes Dixon. And yes, easier is great when it comes to making dinner or doing your taxes, but not when you’re talking about ab exercises. “I see so many people putting their hands too close together or their shoulders way behind their wrists – or their booty sticks up!” she says. All of these alignment snafus mean you’re not giving your core the workout it needs. Luckily, fixing your form is a cinch: Make sure your shoulders are directly over your wrists and your hips are in line with your shoulders.